1. Field of the Invention
This invention relates to closed cycle refrigeration systems and particularly to a defrost arrangement therefor. Still more particularly, the invention relates to a new and improved gas defrost arrangement for a closed cycle refrigeration system.
2. Description of the Prior Art
In a conventional closed cycle refrigeration system, gaseous refrigerant is compressed to a relatively high temperature and relatively high pressure. This compressed gaseous refrigerant is condensed to a liquid at or close to the compressor discharge pressure. The liquid refrigerant is evaporated at a substantially lower pressure in evaporating coils to accomplish the desired cooling effect at the evaporators; this evaporated refrigerant is returned to the compressor and recompressed to complete and continue the refrigeration cycle.
The refrigerant absorbs a substantial amount of heat during the evaporation stage which is dissipated by the condenser as a waste byproduct of the refrigeration cycle. Refrigerating systems employing gas defrost utilize a certain amount of this extra heat by channeling some of the hot compressed gaseous refrigerant back to the evaporators where this heat is given up by the gaseous refrigerant to defrost the evaporators.
In one type of conventional gas defrost system, the superheated gaseous refrigerant is periodically channeled directly from the compressor output into one or more selected evaporator coils to melt the frost accumulated thereon. Examples of such systems are shown in Friedman et al. U.S. Pat. No. 3,138,007 issued June 23, 1954 and Blake U.S. Pat. No. 3,150,498 issued Sept. 29, 1964.
Other conventional gas defrost systems remove the superabundance of sensible heat from the compressor discharge gas so that the defrost gas conveyed to the selected evaporator(s) to be defrosted is at or close to the saturation temperature of the refrigerant. Examples of such systems are shown in Latter U.S. Pat. No. 2,895,306 issued July 21, 1959 (heat exchange between superheated gaseous refrigerant from compressor discharge and evaporated gaseous discharge from suction side of closed system); and Quick U.S. Pat. No. 3,343,375 issued Sept. 26, 1967 (mixing of condensed liquid refrigerant with superheated gaseous refrigerant from the compressor discharge or direct use of receiver gas).
Still other prior art systems remove both superheat and latent heat from the defrosting refrigerant so that only condensed liquid refrigerant is conveyed to the evaporator to be defrosted (Decker et al., U.S. Pat. No. 3,195,321 issued July 20, 1965); still other systems increase the heat content of the defrost gas by means of external electric heaters and the like (Beckwith U.S. Pat. No. 3,147,602 issued Sept. 8, 1964).
Conventional gas defrost arrangements for closed cycle refrigeration systems generally fall into two main categories: (1) those in which superheated gaseous refrigerant approximately at or above the compressor discharge temperature is conveyed to the evaporator to be defrosted; and (2) those in which substantially all of the superheat is removed and the defrost refrigerant conveyed to the evaporator to be defrosted is substantially at saturation temperature or below saturation in condensed liquid form. Both types of known gas defrost systems have certain disadvantages.
In commercial refrigeration systems, one or more compressors (connected, for example, in parallel relation) are located, along with at least one receiver tank and associated valving and manifolding, in a central location, often referred to as the "compressor room". The condenser(s), normally of the air cooled type, are usually remotely located at the exterior of the building, at the side or on the roof, e.g. about 40 to 100 feet from the compressor room. Refrigerated case evaporator coils associated with each of a plurality of refrigerated food storage cases and the like are located remote from the compressor room at various locations within the store. Conduits of substantial length (e.g. between about 50 and 300 feet) connect each evaporator with the liquid refrigerant source in the compressor room. Thus each evaporator is connected to the closed refrigeration system by a pair of substantially long conduits; one such conduit extends between the liquid manifold and the evaporator on the high side and the other between the evaporator and the suction manifold on the low side. In refrigerating systems utilizing gas defrost, the low side conduit is connected at its compressor room end to a three-way valve which in turn has its other two ports connected to the suction manifold and to a defrost gas manifold. These connecting conduits alternately carry refrigerant in one direction in the refrigerating mode and in the opposite direction in the defrosting mode.
In known refrigerating systems employing the compressor discharge as the defrost medium, these connecting conduits can be subject to wide temperature variations; the longer conduits (e.g. in the 150-300 foot range) experience substantial thermal expansion and contraction. Failure to take such changes into account during installation of a system can result in line damage and/or breakage over a period of time. Also, the superheated defrost gas can produce undesired excess heating in the region of the evaporators whereby food products which may be located close to the evaporators may be warmed and wholly or partially defrosted themselves.
To avoid the above-mentioned problems associated with superheated gas defrosting, systems were developed whereby the temperature of the defrost gas was reduced to saturation before being conducted through the connecting conduits to the evaporator(s) to be defrosted. In commercial operations, such systems included mixing the compressor discharge gas with condensed liquid refrigerant or tapping the saturated gas in the receiver tank directly. However, in any fluid flow system in which heated fluid flows through a conduit of substantial length (such as the connecting conduits between the evaporators and the compressor room), heat is given up by the fluid as it traverses the length of the conduit so that the temperature of the fluid exiting from the conduit is measurably lower than the temperature of the fluid entering the conduit. In gas defrost systems when the temperature of the defrost gas is reduced to substantially saturation temperature in the compressor room, a substantial portion of the defrost gas will give up its latent heat as it traverses the connecting conduit so that at the evaporator end of the conduit, this substantial portion of defrost refrigerant will be in its liquid phase. The result is two-fold: (1) the condensed liquid refrigerant has only sensible heat to give up for defrosting (having fewer BTUs per pound than latent heat); and (2) additional liquid refrigerant must be added to the system to account for this condensation and avoid excessive pressure drops in the system thereby resulting in further inefficiencies.
The above discussed disadvantages of known gas defrost systems are overcome by the present invention, which has as principal objects:
(1) The creation of an additional refrigeration load in a central refrigeration system so that a larger percentage of the system may be defrosted at one time; and PA1 (2) Reduction of the temperature of the refrigerant supplied to the defrost gas manifold to a specified temperature advantageously above saturation but well below compressor discharge temperature; and PA1 (3) Maintenance of the specified temperature during substantially the entire defrost cycle.
It is a further object of the invention to provide a gas defrost system for a closed cycle refrigeration system which is capable of absorbing excess liquid refrigerant introduced into the refrigeration cycle during the defrosting of one evaporator or section of evaporators.
It is a still further object of the invention to provide a heat exchange arrangement whereby excess heat may be removed from compressor discharge gas in an amount sufficient to reduce the temperature thereof to a specified amount above the saturation temperature of that refrigerant at about the compressor discharge pressure such that the temperature differential above saturation will remain substantially constant through variations in the compressor discharge pressure.
The above and other objects of the present invention are incorporated into a multiple refrigerating system involving two or more evaporators in which the compressor is remote from the evaporators and less than all of the evaporators are defrosted at a given time.